(1) Field of the Invention
The present invention relates to an adsorptive separation process for continuously separating a specific isomer of a trisubstituted benzene (such as 2,6-dichlorotoluene or other specific dichlorotoluene isomer) from a mixture containing at least two isomers of the trisubstituted benzene (such as a mixture of DCT isomers) by using a simulated moving bed.
(2) Description of the Prior Art
When a single isomer is prepared and recovered, the following problems inevitably arise in all conventional processes. In the first place, the formation ratio between an isomer formed as a by-product and an isomer to be recovered is not in agreement with the demanded ratio between the two isomers, and the by-product is a surplus and thus conventional processes are not preferred from the resource-saving viewpoint. In the second place, the difference of the boiling point between an isomer formed as by-product and an isomer to be recovered is so small that it is difficult to separate these isomers from each other by ordinary rectification, with the result that the separation process becomes complicated and a large quantity of energy is consumed for the process. The first problem may be solved by adoption of a process in which an isomer formed as a by-product is isomerized under isomerization reaction conditions and is converted to an isomer to be recovered. The second problem may be solved by adoption of a recovery process utilizing an adsorptive separation technique or crystallizing separation technique. However, solutions of both the problems are still insufficient.
For example, 2,6-dichlorotoluene (dichlorotoluene is hereinafter referred to as "DCT" for brevity) is an important intermediate for dyes, medicines and agricultural chemicals. However, since boiling points of DCT isomers are very close to one another, it is very difficult to separate 2,6-DCT from a DCT isomer mixture by rectification. Accordingly, 2,6-DCT is ordinarily prepared on an industrial scale, for example, by a process comprising dichlorination of p-toluene-sulfonic acid and subsequent desulfonation. However, a product having a high purity can hardly be obtained according to this process, and the process is not preferred from the economical viewpoint.
Furthermore, 3,5-DCT is expected as an intermediate for a novel agricultural chemical, but no industrial preparation process has been established because in the nuclear chlorination reaction of chlorotoluene the ortho- and para-orientations are strong and 3,5-DCT is hardly formed and because even if 3,5-DCT is formed by isomerization of other DCT isomers, the boiling point of 3,5-DCT is substantially equal to the other DCT isomers formed as by-products such as 2,4-DCT, 2,5-DCT and 2,6-DCT (having boiling points of about 200.degree. C.) and it is substantially impossible to separate 3,5-DCT having a high purity by rectification.
U.S. Pat. No. 4,254,062 proposes a process for separating a specific DCT isomer from a DCT isomer mixture by using a zeolite of the faujasite type as an adsorbent. However, if it is intended to separate 2,6-DCT as the extract component, that is, as the highly adsorbable component, from a DCT isomer mixture, since the adsorbability of 2,3-DCT and/or 2,5-DCT to a zeolite of the faujasite is substantially equal to or higher than that of 2,6-DCT, it is substantially impossible to separate 2,6-DCT having a high purity.
Furthermore, the above mentioned U.S. patent does not refer to adsorptive separation of 3,5-DCT but discloses a separation technique to be applied to a system which does not substantially contain 3,5-DCT which is a nuclear chlorination product of toluene or chlorotoluene. Accordingly, it is not known whether the adsorptivity of a faujasite type zeolite adsorbent to 3,5-DCT is higher or lower than that of the adsorbent to other DCT isomers or whether or not it is possible to substantially separate 3,5-DCT having a high purity from a DCT isomer mixture by using this adsorbent.